The present invention relates generally to phase shifter circuitry. More specifically, the present invention relates to switched-line phase shifters using parallel coupled line sections.
A basic component in microwave/millimeter wave circuits is the differential phase shifter. Differential phase shifters are commonly implemented using a switched-line configuration in which switching devices are used to switch a signal between alternate transmission paths. The alternate transmission paths, in turn, have different electrical lengths, and thus there is a difference in relative signal phase between signals propagated through the alternate transmission paths. For example, if a first transmission line has an electrical length of .lambda./2 (where .lambda. is the wavelength of the signal) and a second transmission line has an electrical length of .lambda./4, the differential phase shift between the two transmission paths is .lambda./4 (or 90.degree.).
One problem with conventional switched-line phase shifters incorporating non-coupled transmission lines is that the differential phase shift varies with signal frequency. For example, a first transmission line with an electrical length of .lambda..sub.1 /2 at a frequency of 3 GHz may have an electrical length of .lambda..sub.2 /4 at 1.5 GHz. Likewise, a second transmission line with an electrical length of .lambda..sub.1 /4 at a frequency of 3 GHz may have an electrical length of .lambda..sub.2 /8 at 1.5 GHz. Thus, while the differential phase shift between the two transmission lines at 3 GHz is .lambda./4, the differential phase shift between the same two transmission lines at 1.5 GHz is .lambda./8.
In response to the need to maintain a single phase shift over a range of frequencies, switched-line phase shifters utilizing parallel coupled-transmission lines (hereinafter "Schiffman sections") have been developed. Such phase shifters are described by B. M. Schiffman in the paper entitled "A New Class of Broad-Band Microwave 90-Degree Phase Shifters," IRE Transactions on Microwave Theory and Technique, April 1958, pages 232-237.
One problem with switched-line phase shifters, including Schiffman-type phase shifters using series switches, is that when the effective electrical length of the switched-off transmission path is an integer multiple of 180.degree. (half the wavelength of the operating frequency), a resonance is established in the switched-off path. The resonance results from the practical implementation of switching devices that have leakage capacitance. Although the switched-off path is theoretically isolated from the external network, in actuality the switched-off path is capacitively coupled to the external network. Since the switched-off path is coupled to the rest of the network (including the switched-on transmission path), the effects of the switched-off path resonance are seen in the performance of the switched-on path as well. In particular, the resonance results in phase shifter operating points of high signal attenuation (also known as isolation points) at the operating frequencies associated with the points of resonance.
Various techniques have been proposed to reduce the resonance effect. For example, the use of transfer switches instead of standard single pole double throw (hereinafter "SPDT") switches to switch between transmission paths has been explored. In a transfer switch implementation, when a transmission path is switched off, it is connected to a load with a matching characteristic impedance. Although the resonance problem can be reduced through the use of transfer switches, the performance of a switched-line phase shifter using this loading technique suffers the disadvantage of considerable driver circuit complexity. In addition, the bandwidth of such a phase shifter is limited due to the RF properties of the associated switching devices and load circuitry.
Another technique that has been explored, for example in the paper entitled "An Octave-Band Switched-Line Microstrip 3-b Diode Phase Shifter," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-21, No. 7, July 1973, pages 444-449, is the use of shunt switches instead of series switched in the switched-line configuration. Though theoretically appealing, the performance of switched-line phase shifters utilizing shunt switches in practice is severely degraded due to the parasitic reactance present in practical switching devices. Furthermore, they use quarter-wave transformers to isolate the switched-off path at the input and output ports of the switched-line phase shifter. However, the quarter-wave transformers cannot be operated over a wide bandwidth, and thus limit the effective bandwidth of the phase shifter.
A need has long existed for a wide-bandwidth switched-line phase shifter with relatively constant phase-frequency characteristics that eliminates isolation points over the design operating frequency range without incorporating complex circuitry or introducing substantial performance degradation.